1. Field of the Invention
The present invention relates to a semiconductor light emitting device, and more particularly to a gallium nitride based semiconductor light emitting device.
2. Description of the Related Art
Nitride based semiconductors are extremely attractive for a blue color laser device. In Japan Journal Applied Physics vol. 36, 1997, pp. 1568-1571, Nakamura et al. reported a continuous emission life-time for over 10,000 hours at 2 mW and at room temperature. FIG. 1 is a fragmentary cross sectional elevation view of a conventional laser diode. A GaN film 102 is formed on a sapphire substrate 101. Stripe-shaped SiO2 masks 103 are formed on the GaN film 102.
Further, a gallium nitride based multilayer structure is selectively grown by use of the stripe-shaped SiO2 masks 103, whereby the gallium nitride based multilayer structure includes low dislocation density regions 104 and high dislocation density regions 116 which are marked by hatching. The high dislocation density regions 116 can be obtained by the normal epitaxial growth in the vertical direction from the GaN film 102. The low dislocation density regions 104 can be obtained by the epitaxial lateral overgrowth over the stripe-shaped SiO2 masks 103. A p-electrode 105 is provided on the low dislocation density region 104. The GaN film 102 has a high through-dislocation density.
The through-dislocations extend to the high dislocation density regions 116 as well illustrated in the hatched region of FIG. 1. The through-dislocations do not extend to the low dislocation density region 104. The high dislocation density region 116 has a high dislocation density of not less than 1×1012 m−2. The low dislocation density region 104 has a low dislocation density of less than 1×1011 m−2. In the vicinity of the center of the SiO2 mask 103, the epitaxial lateral overgrowths from the peripheral sides of the SiO2 mask 103 toward the center of the SiO2 mask 103 experience collision, whereby another dislocation appears, for which reason the low dislocation density region 104 has another high dislocation density region over the center of the SiO2 mask 103.
The p-electrode 105 is provided on the low dislocation density region 104, so that a current is injected into the low dislocation density region 104 to avoid any deterioration of the laser device due to the dislocation, and to obtain a possible long life-time.
Over an Si-doped n-GaN epitaxial lateral overgrowth substrate 106, an Si-doped n-type In0.1Ga0.9N layer 107 is formed. An n-type cladding layer 108 is formed on the Si-doped n-type In0.1Ga0.9N layer 107, wherein the n-type cladding layer 108 comprises 120 periods of Si-doped n-type GaN layers having a thickness of 2.5 nanometers, and undoped Al0.14Ga0.86N layers having a thickness of 2.5 nanometers. An Si-doped n-type GaN optical confinement layer 109 having a thickness of 0.1 millimeter is formed on the n-type cladding layer 108.
A multiple quantum well active layer 110 is formed on the Si-doped n-type GaN optical confinement layer 109, wherein the multiple quantum well active layer 110 includes Si-doped n-type In0.15Ga0.85N well layers having a thickness of 3.5 nanometers and Si-doped n-type In0.02Ga0.98N potential barrier layers having a thickness of 10.5 nanometers. An Mg-doped p-type Al0.2Ga0.8N cap layer 111 having a thickness of 20 nanometers is formed on the multiple quantum well active layer 110. An Mg-doped p-type GaN optical confinement layer 112 having a thickness of 0.1 millimeter is formed on the Mg-doped p-type Al0.2Ga0.8N cap layer 111.
A p-type cladding layer 113 is formed on the Mg-doped p-type GaN optical confinement layer 112, wherein the p-type cladding layer 113 comprises 120 periods of Mg-doped p-type GaN layers having a thickness of 2.5 nanometers, and undoped Al0.14Ga0.86N layers having a thickness of 2.5 nanometers. An Mg-doped p-type GaN contact layer 114 having a thickness of 0.05 millimeters is formed on the p-type cladding layer 113. A dry etching process is carried out to form a ridge structure. A p-electrode 105 and an n-electrode 115 are formed, wherein the p-electrode 105 comprises laminations of Ni-layer and Au-layer, whilst the n-electrode 115 comprises laminations of Ti-layer and Al-layer.
In Japan journal Applied Physics vol. 36, 1997, pp. L899-902 and NEC Research and Development vol. 41, 2000, No. 1, pp. 74-85, the present inventors addressed that the facet-initiated epitaxial lateral overgrowth method was used to reduce the dislocation density over entire region. In the facet-initiated epitaxial lateral overgrowth method, the strip-shaped SiO2 masks are formed on the GaN layer over the sapphire substrate, and then a hydride vapor phase growth is used to cause the extending direction of the through-dislocations to be curved, whereby the dislocation density is relaxed, no high dislocation density region is formed. Thus, a low dislocation density n-GaN substrate can be prepared.
FIG. 2 is a fragmentary cross sectional elevation view of another conventional semiconductor laser device. An n-type cladding layer 122 was formed on a top surface of the n-GaN substrate 121, wherein the n-type cladding layer 122 comprises an Si-doped n-type Al0.1Ga0.9N layer having a silicon impurity concentration of 4×1017 cm−3 and a thickness of 1.2 micrometers. An n-type optical confinement layer 123 was formed on a top surface of the n-type cladding layer 122, wherein the n-type optical confinement layer 123 comprises an Si-doped n-type GaN layer having a silicon impurity concentration of 4×1017 cm−3 and a thickness of 0.1 micrometer.
A multiple quantum well active layer 124 was formed on a top surface of the n-type optical confinement layer 123, wherein the multiple quantum well active layer 124 comprises three In0.2Ga0.8N well layers having a thickness of 3 nanometers and Si-doped In0.05Ga0.95N potential barrier layers having a silicon impurity concentration of 5×1018 cm−3 and a thickness of 5 micrometers.
A cap layer 125 was formed on a top surface of the multiple quantum well active layer 124, wherein the cap layer 125 comprises an Mg-doped p-type Al0.2Ga0.8N layer. An optical confinement layer 126 was formed on a top surface of the cap layer 125, wherein the optical confinement layer 126 comprises an Mg-doped p-type GaN layer having a magnesium impurity concentration of 2×1017 cm−3 and a thickness of 0.1 micrometer. A p-type cladding layer 127 was formed on a top surface of the optical confinement layer 126, wherein the p-type cladding layer 127 comprises an Mg-doped p-type Al0.1Ga0.9N layer having a magnesium impurity concentration of 2×1017 cm−3 and a thickness of 0.5 micrometers. A p-type contact layer 128 was formed on a top surface of the p-type cladding layer 127, wherein the p-type contact layer 128 comprises an Mg-doped p-type GaN layer having a magnesium impurity concentration of 2×1017 cm−3 and a thickness of 0.1 micrometer.
Those layers 122, 123, 124, 125, 126, 127, and 128 were formed by a low pressure metal organic vapor phase epitaxy method under a pressure of 200 hPa. A partial pressure of the ammonium gas for nitrogen source was maintained at 147 hPa. TMG (trimethyl gallium) was used for the Ga source material. TMA (trimethyl aluminum) was used for the Al source material. TMI (trimethyl indium) was used for the In source material. The growth temperature was maintained at 1050° C. except when the InGaN multiple quantum well active layer 124 was grown. In the growth of the InGaN multiple quantum well active layer 124, the growth temperature was maintained at 780° C.
A dry etching process was then carried out to selectively etch the p-type cladding layer 127 and the p-type contact layer 128 thereby forming a mesa structure 129. A silicon dioxide film 130 was formed on the mesa structure 129 and the upper surfaces of the p-type contact layer 128. The silicon dioxide film 130 was selectively removed from the top surface of the mesa structure 129 by use of an exposure technique, whereby the top surface of the p-type contact layer 128 was shown and a ridged structure was formed.
An n-type electrode 131 was formed on a bottom surface of the substrate 121, wherein the n-type electrode 131 comprises laminations of a titanium layer and an aluminum layer. A p-type electrode 132 was formed on a top surface of the p-type contact layer 128, wherein the p-type electrode 132 comprises laminations of a nickel layer and a gold layer. The above structure was then cleaved to form first and second facets. Two samples were prepared. In the first type sample, both the first and second facets were then coated with a highly reflective coat of a reflectance factor of 95%, wherein the highly reflective coat comprises laminations of titanium dioxide film and silicon dioxide film. In the second type sample, only the second facet was then coated with a highly reflective coat of a reflectance factor of 95%, wherein the highly reflective coat comprises laminations of titanium dioxide film and silicon dioxide film. The first facet was uncoated. The first type sample showed a threshold current density of 1.5 kA/cm2. The second type sample showed a threshold current density of 3.0 kA/cm2.
For the nitride based semiconductor blue color laser device, the InGaN quantum wells are provided in the active layer. It was not easy to prepare uniform InGaN amorphous films in the crystal growth. The InGaN quantum well active layer has compositional fluctuation. In accordance with the conventional common sense, the compositional fluctuation deteriorates the device performances or characteristics. The issue for the prior art was how to eliminate the compositional fluctuation.
In Applied Physics Letter vol. 71, p. 2346, 1997, Chichibu et al. reported results of considerations for the indium-compositional fluctuation and the carrier diffusion length basd on the observation of the cathode luminescence image to the InGaN quantum wells. It was presumed even the dislocation density was higher in order than the conventional one, a high luminescence efficiency could be obtained because potential fluctuations of electrons and holes caused by the indium compositional fluctuation localize carriers to prevent the carriers to be captured into the non-radiation centers. If this presumption is correct, it may be expectable that the increase of the compositional fluctuation without reducing the dislocation density improves the luminescent efficiency. Actually, however, the device performances depend on the optical gain, for which reason a variation in the state density due to the indium compositional fluctuation causes a large change in the optical gain.
In Applied Physics Letter, vol. 71, p. 2608, 1997, Chow et al. addressed that the compositional fluctuation makes the gain spectrum wide, whereby the gain peak is lowered and the threshold current density is dropped.
Japanese laid-open patent publication No. 11-340580 discloses to control the indium compositional fluctuation, wherein the compositional uniformity is improved to prevent the multi-wavelength laser emission. The compositional uniformity is measurable from the photo-luminescence peak wavelength distribution. This publication also discloses the following prior art. An InGaN non-amorphous large region is present. A small indium compositional region with indium index of not more than 0.2 may cause a compositional isolation due to the increase in the indium index.
If the InGaN layer has an indium index of about 0.15, then a half-width of the photo-luminescence spectrum in the macroscopic scale in the order of about 200 micrometers is extremely large due to non-uniformity of the crystal structure with the compositional isolation, and the half-width is at least 150 meV The publication discloses, as the prior invention, that a SiC substrate is used to adjust the crystal growth rate for reducing the photo-luminescent peak wavelength distribution to about 90 meV, thereby preventing the multi-wavelength laser emission.
The above Japanese publication is silent on the important issue of further reduction of the photo-luminescent peak wavelength distribution from 90 meV. The effect of the indium compositional fluctuation to the device performances had not sufficiently been clarified. The fundamental question on the compositional fluctuation of the InGaN quantum well in the active layer had also not been sufficiently clarified.
The most attractive application of the blue color laser device is a light source for wiring and reading to the optical disks such as DVD. For the read operation, about 3 mW output of the blue color laser beam is necessary. For the write operation, about 30 mW output of the blue color laser beam is necessary. If the laser device is used for DVD-RAM, then the laser device should have the high output performance of about 30 mW, for which reason one of the facets of the laser device is coated with a highly reflective coating. If the laser device is used for DVD-ROM, then the laser device should have the low output performance of about 3 mW, for which reason both the facets of the laser device are coated with a highly reflective coating.
Whereas it is desired that the high output laser device should have a life-time of at least 5000 hours at 30 mW and at 70° C., the actually realized life-time was at most 500 hours at 30 mW and at 60° C., which was reported by Nakamura et al. in JSAP International No. 1, pp. 5-17, 2000. The reduction in the driving current at 30 mW makes the life-time long, for which reason it is desirable to reduce the threshold current of the high output laser.
The low output laser device may be used for the portable DVD player with a battery. For this low output laser device, it is desirable to reduce the low power consumption. For this purpose, it is desirable to reduce the threshold current.
In the above circumstances, the development of a novel nitride based semiconductor light emitting device free from the above problems is desirable.